Tree Cavities: To Fill or not to Fill

It is common for old trees to develop cavities in their trunks. In fact, it is fairly rare to find a very old tree that is not hollow due to wood decay. Most of wood in a tree is dead xylem tissue. If decay organisms enter a trunk, decay can progress undetected for decades. A tree can live for hundreds of years with a hollow trunk as long as it has a healthy cylinder of wood and bark 3 or 4 inches in thickness. In most cases, a hollow tree does not constitute hazard, but the condition is usually discovered when the tree falls or is damaged by high winds. Often, when a large branch falls from a tree, it is apparent that decay has hastened breakage that would eventually have occurred due to mechanical weakness associated with poor branching structure. This illustrates the importance of regular pruning during the establishment phase of a new tree to create a strong framework of branches.

Panic is the most common reaction that I have observed when individuals first discover a cavity in the trunk of a beloved, old tree. This is sometimes intensified when he or she also observes carpenter ants in and around the opening. The first question that comes during the panicked call to the Extension Center is, “What should I spray to kill the bugs?” Following close behind comes; “What should I use to fill the hole?” When I get this type of call, my first task is to convince the concerned tree lover that the ants are not going to kill the tree. Carpenter ants do not work in living wood tissue. They are, however, commonly found tunneling in decaying wood to create a nest. However, unlike termites, they do not eat wood and are unlikely to do significant damage to the living wood of a tree or contribute much to loss of structural integrity. If a nest is located near a wooden structure, it may be worth treating it with a labeled insecticide to eliminate the possibility that they may move to the building. Otherwise, the ants can be ignored. See http://muextension.missouri.edu/explore/agguides/pests/g07423.htm for more information on carpenter ants.

The question of whether or not to fill tree cavities has been debated for decades. The rationale cited for filling has been that it may prevent moisture, insects and animals from entering the tree, thereby preserving structural integrity. Thirty years ago it was fairly common to see large cavities filled with bricks and mortar or concrete. While these treatments look impressive, the general consensus among tree experts is that they probably do more harm than good, grinding away at the tree as it moves in the wind. In his book, “Tree Maintenance,” P. P. Pirone suggests that in nearly all cases, cavity filling has only cosmetic value. However, if it is done, the punky wood should be removed carefully before filling to avoid damaging surrounding, healthy tissue and promoting spread of the decay. In his book “Arboriculture,” Richard Harris states, “From the standpoint of tree structure and health, the reasonable course of action is simply to leave cavities alone.” Both Pirone and Harris caution against boring holes in the trunk at the lower end of a trunk cavity to drain water. This treatment is guaranteed to breach healthy tissue that may have been “compartmentalized” by the tree, opening it up to infestation by decay organisms.

In addition to the logical arguments against cavity filling, there are some practical impediments. In cases where water is likely to collect in a cavity, it is sometimes recommended that the void be filled with expandable polyurethane foam. During a recent storm, approximately half of a huge sugar maple tree in a neighbor’s yard blew down, leaving a gaping hole in the remaining trunk. After the fallen branch (30” inches in diameter) was hauled away, I observed the tree and suggested the expandable foam approach. However, after checking for local suppliers of polyurethane and businesses willing and able to fill a large cavity with the material, it became apparent that this is not a viable option in Columbia, Mo. The only foam that I could find came in small cans suitable for filling cavities around windows and doors, but not for filling a 100-gallon hole in a tree. The tree companies that I called must have read Harris’s book. They indicated that they did not fill cavities because they are best left untreated.

After cogitating on the situation for a few days, I came up with a compromise suggestion for my neighbor. Perhaps she could create a water-shedding barrier by tacking window screen over the hole and covering it with a layer of foam from the small cans that are available locally. The covering could then be painted to match the color of the bark. Several days ago, my neighbor stopped by to inform me that she had had someone climb a ladder to take a closer look at the storm damage. It turned out that the cavity that we could see from the ground only extended upward and not below the site where the branch broke off. Since water was not likely to collect, she and her husband decided to take the Harris approach and leave the tree as it is.

The debate among tree experts on how to treat wounds and cavities will likely continue for several more decades. However, one thing most can agree upon is that increasing the vigor of a tree will help prevent the spread of decay. Moderate fertilization, irrigation during drought, mulching and aerification via holes augered into the soil around the tree may be all a tree needs to help it halt the progress of decay in its trunk. It seems that cavity filling is primarily done for the benefit of the tree steward.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630